Expressing Concentration — Part 2
Molarity, molality, and why temperature matters
A chemistry lab prepares 1 M NaCl solution at 25°C and stores it in a sealed bottle in a freezer at 4°C. When retrieved later, the label still says "1 M NaCl" but a student recalculates the molarity using the solution volume at 4°C and gets a slightly different number. Did the solution's concentration change? Is the label wrong?
Ocean water has a salinity of about 35 g per kilogram of sea water. Oceanographers express this as 35 per mille (‰) — 35 parts per thousand by mass. This is effectively molality (grams per kilogram), not molarity — because sea water temperatures vary from −2°C in polar regions to +30°C near the equator. A concentration unit based on mass doesn't drift with temperature. The same logic drives every laboratory situation where temperature isn't controlled precisely.
Molarity (M)
Molarity is the number of moles of solute dissolved per litre of solution.
Or equivalently, if you know the mass of solute ( in grams) and molar mass ():
Units: mol/L, also written mol L⁻¹ or M.
Key limitation: Molarity changes with temperature because volume changes with temperature.
Molality (m)
Molality is the number of moles of solute dissolved per kilogram of solvent.
where = mass of solute in g, = molar mass of solute, = mass of solvent in g.
Units: mol/kg, also written mol kg⁻¹ or m.
Key advantage: Molality is temperature-independent because mass of solvent does not change with temperature.
Q1.3.65 g of HCl (M = 36.5 g/mol) is dissolved in water to make 500 mL of solution. What is the molarity?
A chemistry lab prepares 1 M NaCl solution at 25°C and stores it in a sealed bottle in a freezer at 4°C. When retrieved later, the label still says "1 M NaCl" but a student recalculates the molarity using the solution volume at 4°C and gets a slightly different number. Did the solution's concentration change? Is the label wrong?
Ocean water has a salinity of about 35 g per kilogram of sea water. Oceanographers express this as 35 per mille (‰) — 35 parts per thousand by mass. This is effectively molality (grams per kilogram), not molarity — because sea water temperatures vary from −2°C in polar regions to +30°C near the equator. A concentration unit based on mass doesn't drift with temperature. The same logic drives every laboratory situation where temperature isn't controlled precisely.
Molarity (M)
Molarity is the number of moles of solute dissolved per litre of solution.
Or equivalently, if you know the mass of solute ( in grams) and molar mass ():
Units: mol/L, also written mol L⁻¹ or M.
Key limitation: Molarity changes with temperature because volume changes with temperature.
Molality (m)
Molality is the number of moles of solute dissolved per kilogram of solvent.
where = mass of solute in g, = molar mass of solute, = mass of solvent in g.
Units: mol/kg, also written mol kg⁻¹ or m.
Key advantage: Molality is temperature-independent because mass of solvent does not change with temperature.
Q1.3.65 g of HCl (M = 36.5 g/mol) is dissolved in water to make 500 mL of solution. What is the molarity?